Interpretive Summary: The soybean aphid, a common soybean insect pest in Asia, was first found in Wisconsin during July 2000. Once established on a plant, soybean aphid populations can increase logarithmically via asexual reproduction. Soybean aphids also can develop winged form body morphology during summer months which allows the insect to spread through the air to infest large geographic areas, even in areas where the aphid does not overwinter. By 2003, the insect was found in 21 U.S. states and 3 Canadian provinces and is now considered to be a major soybean insect pest in the north central US. Predictions are that this insect will expand its range to include all regions of the US where soybeans are grown. Because the soybean aphid is a relatively new pest in the U.S., a comprehensive integrated pest management (IPM) approach to manage aphid populations has yet to be developed. We felt that a controlled-infestation field study of how aphid populations affect of soybean growth, yield, and seed composition would be a step toward the development of IPM systems for this invasive insect pest. This 2-yr field study developed stage-specific soybean aphid economic injury levels that allow growers to accurately manage soybean aphids by considering the market value of soybean, aphid control costs, and soybean yield potential. Relating these aphid population parameters at the plant growth stages studied enables producers to make informed decisions about the need for and timing of rescue chemical treatments.

Technical Abstract:
Stage-specific economic injury levels form the basis of an integrated pest management approach for soybean aphid (Aphis glycines Matsumura) population management in soybeans (Glycine max L.). Experimental objectives were to develop a procedure for calculating economic injury levels of the soybean aphid specific to the R2 (full bloom), R4 (full pod), and R5 (beginning seed) soybean development stages using the law of the diminishing increment regression model. Aphid population growth over time appeared to follow the symmetrical bell-shaped and logistic growth curve models. Peak aphid population levels and rates of increase were observed to occur on the R5 development stage and then declined sharply thereafter. Highest peak aphid populations observed were 21626 aphids plant-1 for infestations starting at V5, and 6446 aphids plant-1 for infestations starting at R2. Highest maximum aphid-days plant-1 recorded were 537217 for V5-introduced aphids and 148609 aphid-days plant-1 for R2-introduced aphids. On average, the calculated maximum possible yield loss was 74% for soybean aphid infestations starting at the V5 (five node) stage and 35% for aphid infestations starting at the R2 stage. Interrelationships among the current or predicted market value of soybean, cost of controlling the aphids on the field, and the yield potential of the soybean field were considered in the calculations of the stage specific economic injury levels. Practical examples for calculating stage specific economic injury levels are presented. Use of these stage-specific economic injury levels may enable growers to manage soybean aphids more accurately.